It is common when making steel to take molten iron from a blast furnace, subject it to desulfurization, introduce it into a basic oxygen furnace to remove carbon, and to then continuously cast the resultant liquid product. As a practical matter, it is desired to complete the desulfurization process without undue delay, in order not to interrupt downstream processing. If there is an interruption in flow of materials and the ribbon of continuous cast material becomes broken, it costs $100,000 or more to restart the ribbon. Therefore, it is essential that the desulfurization of the iron from the blast furnace continue without significant interruption.
In a single lance, single conduit prior art design of the type previously commercialized by ESM II and others, two feed vessels are commonly utilized, one for magnesium reagent powder mixtures (typically powdered in the range of 0.2 to 1.0 millimeters diameter) and one for a pulverized reagent, which may be lime, carbide, or other mixture of products, (typically pulverized to about 75 micron diameter), the pulverized reagent being used to transport the magnesium reagent. (While two vessels are commonly used, in some situations there may be a single vessel, or more than two vessels.) Each of the vessels is pressurized. At the bottom of each of the two vessels is an orifice. The orifice may be a variable valve of the type disclosed in U.S. Pat. No. 5,108,075, or it may be fixed. If fixed, flow rates may be varied by varying the pressure in the vessel, or by changing the orifice. When fixed orifices are employed, it is also necessary to employ a gate valve or the equivalent.
Initially, an inert gas under pressure, which is typically referred to as transport gas, will be introduced into a tube below the orifice in the pulverized reagent vessel to initiate flow of the pulverized reagent. The transport gas will initially flow from a location below the orifice of the pulverized reagent vessel to a location below the orifice of the magnesium reagent vessel, so the pulverized reagent can pick up the magnesium reagent, and transport it to a lance. Once flow has been established, further use of transport gas is minimized in the transport system. Typically, the pulverized reagent and magnesium reagent powder mixture will be mixed in a 3:1 ratio, i.e., 75 lbs. of pulverized reagent per minute to 25 lbs. of magnesium reagent per minute, although other ratios may be employed. This mixed product will be introduced into a ladle which may vary in size to hold approximately 100-300 metric tons of iron. This mixed product is introduced into the bottom of the ladle via a lance into a "reaction zone" where the magnesium reagent reacts with sulfur within the molten iron to drive off the sulfur. The lance includes a monolithic refractory element formed typically about a 3"diameter round or square section structural tube which in turn receives a 1/2", 3/4", or 1" pipe, the magnesium reagent and pulverized reagent flowing through the pipe within the structural tube.
In the single lance, single conduit operation just described, there is a maximum rate at which the magnesium reagent can be introduced into the single reaction zone. This is because magnesium has a high vapor pressure. If introduced too fast there may be undesirable splashing and turbulence resulting in loss of iron, and the efficiency of the reagent is reduced. Therefore, in certain situations, when there is a high initial sulfur content in the molten iron to be processed (for example 0.10% sulfur, with a desired completion percentage of 0.005% sulfur), there may be an undesirable length of time between the start and completion of the desulfurization.
In a prior design, two reaction zones are achieved by providing a lance with a single conduit which terminates at its lower end in a T-fitting. While the reagent will be discharged into two separate reaction zones, to either side of the lance, problems have been encountered. Thus, one of the T's in this type of lance is more likely to become plugged than a single conduit, single discharge lance. When one of the T's becomes plugged, there is only a single desulfurization zones to the side of the lance.
In order to overcome the problems of the single lance, single conduit design, it has been proposed in U.S. Pat. No. 5,188,661 to use a single lance provided with two conduits for the introduction of the desulfurization material. In this single lance, dual conduit design, as can be seen from the drawings of U.S. Pat. No. 5,188,661, each of two steel conduits 16 are disposed within a ceramic body 28 and terminates at a port 24, there being an angled extension formed by an elbow fitting 30. While this single lance dual conduit design provides two reaction zones, permitting the magnesium reagent to be introduced at a faster rate than with a single lance, it shares a problem with the single lance, single conduit design. Thus, in the event a conduit becomes plugged, the lance must be retracted from the molten iron in the ladle, stopping the desulfurization until a new lance can be introduced. While plugging occurs less often than in the T-fitting design, as the pressure in each line tends to keep the discharge port from plugging, plugging still happens. In addition, as the two reaction zones are closely spaced together, there may not be sufficient agitation of all of the iron within the ladle, causing some of the iron not to be fully desulfurized.